It has long been recognized that the sun is a vast source of clean energy and that it can and should be exploited to reduce dependence on fossil fuels. However, systems heretofore available for collecting and utilizing solar energy have not been sufficiently economical to complete effectively with fossil fuels.
One approach that has been taken to enhance cost-effectiveness involves providing (1) a receiver located on a tower or on a hill and (2) a mirror field comprising a plurality of adjustable mirrors situated on the ground and arranged to reflect the sun's rays towards the receiver. The receiver consists of one or more collectors and energy conversion means for collecting the incident radiant energy and converting it to heat. The mirrors, also known as heliostats, are continually repositioned by a servo-tracking system during each day as well as seasonally so as to compensate for relative movement between the sun and earth and thereby contantly direct the reflected solar energy at the receiver. The tracking system may be adapted to compensate for movement of the receiver due to various factors, notably wind loading. The heat energy produced as a consequence of absorption of incident solar radiation by the receiver is recovered by absorbing it in fluid heat transfer medium and passing the latter to or through a heat storage vault or through a consuming device which may take various forms such as a hot water heater or steam producer or turbo-electric generator. It has been suggested that the recovered heat may be stored by absorbing it in a heat storage media of fluid or solid form, e.g. eutectic salts or a bed of stones. Examples of systems embodying centralized energy receivers, mirror fields, servo-controlled sun-tracking heliostats, heat storage tanks, and use of fluid heat transfer media for utilizing absorbed solar energy are disclosed by U.S. Pat. Nos. 4,063,543, 4,044,753, 4,034,735, 4,031,444, 4,021,895, 4,091,495 and 4,013,885.
The prior art solar energy collection and utilization systems do not adequately accommodate and compensate for the fact that the amount of energy from the sun arising at the earth's surface is not constant but will vary with the amount of cloud cover and the time of day. Also the time between sunrise and sundown will vary from day to day. Accordingly if the recovered solar energy is utilized immediately to generate steam from water, a number of disadvantages are incurred. If the steam is used to drive an electrical power generator, the latter must be continually started up and shut down in accordance with the amount of available steam since the latter cannot be stored. This is most uneconomical since the efficiency of a turbine installation is very low when operating under reduced or intermittent loads which lie outside of its design parameters. Furthermore, in such a system the receiver will experience temperatures ranging from that of the boiler feedwater up to the temperatures to which steam is required to be heated for efficient turbine-driving purposes (usually between about 900.degree.-1200.degree. F.), and often it has temperatures even above the usual range of superheated steam. Hence should the incident solar radiation be interrupted, the receiver or at least portions thereof will be cooled from the highest steam temperature in the system down to the lowest boilder feedwater temperature. As a result of this drop, stresses and even shocks are produced in the materials of construction of the receiver. Similar stresses are produced as a result of the large temperature increase which occurs when the receiver is again illuminated with solar radiation.
This problem of thermally-induced stresses exists even if the recovered solar energy is used to generate steam through an intermediate heat transfer medium which permits at least temporary storage of the recovered heat and is capable of being heated high enough to permit production of 900.degree.-1200.degree. F. steam. In this connection it should be noted that using a working fluid other than steam to operate a turbo-generator is not favored since most are designed to be operated by steam.
These thermally-induced stresses, which occur repeatedly, adversely affect the life span of the receiver and may even affect the performance of associated equipment. Since the capital cost of a solar plant of the type described is very high, its useful life must be quite long, typically at least twenty years, in order for the plant to be economically feasible. However, in solar plants of prior design the stresses created by sharp changes in the amount of incident solar radiation are of sufficient magnitude and frequency as to drastically foreshorten the useful life of the receiver or necessitate frequent and expensive shutdowns for repair. Other problems with prior systems employing a centralized energy receiver mounted above and some distance from the heliostats is that dispersion of the reflected beams, due to pointing or other factors, tends to dilute the solar flux so that some of it may not reach the target area of the receiver and/or so that a precise uniformity of flux distribution over the target area of the receiver is not obtainable. While making the target area of the receiver larger will help in intercepting more of the solar energy reflected by the heliostats, it also involves larger receiver construction and maintenance costs. Moreover, since making the target area larger does not provide greater uniformity of flux distribution, an increase in temperature differentials across the receiver (sometimes with an attendant reduction in overall conversion efficiency) is a likely result of increasing the size of the target area. Hence, the problem of stresses is not avoided by making the receiver larger and in fact it is desirable to avoid building larger receivers so as to reduce weight and improve the resistance to wind loading, whereby to achieve a relatively good structural strength-to-cost ratio.